The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Key Applications and Application Protocols            9  TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)                 9  TCP/IP General File Transfer Protocols (FTP and TFTP)                      9  File Transfer Protocol (FTP)                           9  FTP Concepts and General Operation

FTP Concepts and General Operation 1 2 3 4 FTP Control Connection Establishment, User Authentication and Anonymous FTP Access

The standards that define the File Transfer Protocol (FTP) describe its overall operation using a simple conceptual tool called the FTP model. This model defines the roles of the devices that participate in a file transfer, and the two communication channels that are established between them. It also describes the components of FTP that manage these channels, and defines the terminology used for the components. This makes it an ideal place for us to see how FTP works in broad terms.

FTP is a classical client/server protocol, as I mentioned in the overview. However, the client is not called by that name but rather is called the user. The name comes from the fact that the human user that issues FTP commands works on the client machine. The full set of FTP software operating on a device is called a process. The FTP software on the server is called the Server-FTP Process, while the software on the client is the User-FTP Process.

A critical concept in understanding FTP is that while it uses TCP like many other applications, it does not use just one TCP connection for all communication the way most protocols do. The FTP model is designed around two logical channels of communication between the server and user FTP processes:

• Control Connection: This is the main logical TCP connection that is created when an FTP session is established. It is maintained throughout the FTP session and is used only for passing control information, such as FTP commands and replies. It is not used to send files.
  
  
• Data Connection: Each time that data is sent from the server to the client or vice-versa, a distinct TCP data connection is established between them. Data is transferred over this connection. When the file transfer is complete, the connection is terminated.

The reason for having these separate channels is that it provides flexibility in how the protocol is used, as we will see later in this section. It does, however, add complexity to FTP.

FTP Concepts and General Operation 1 2 3 4 FTP Control Connection Establishment, User Authentication and Anonymous FTP Access